The present invention relates generally to diagnostic imaging and, more particularly, to image reconstruction.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
To acquire CT images, generally, one of two projection geometries is used; either a parallel beam geometry or a fan beam geometry. With parallel beam geometry, all x-rays in a projection are parallel to one another. With fan beam geometry, the x-rays at a given projection angle diverge resulting in a fan-like appearance. Most modern CT scanners use fan beam geometry in the acquisition and reconstruction of CT images. Notwithstanding the advantages of fan beam geometry relative to parallel beam geometry, some constraints are placed thereon.
Specifically, fan beam reconstruction was first studied and modeled in the circular scanning case, and later extended to the case of non-circular scanning loci with some constraints. In this regard, conventional fan beam reconstructions are generally based on the Radon formula or the parallel beam reconstruction formula, and completed via the coordinate transformation from parallel beam to fan beam geometry. As a result, conventional fan beam reconstruction techniques are not well-suited for acquiring data from non-still objects, such as during head scans.
For example, a patient having a head CT may be unable to hold the head still and steady during the CT scanning. The movement of the head may result from the patient being young, old, severely injured, or other reasons. Head motion of the patient occurs more often in head perfusion CT. In this technique to measure cerebral blood flow, the same slice of the head is continuously scanned and circumscribed. A specified slice of the brain may be scanned for forty to fifty seconds continuously. A contrast material is injected and the rise and fall of the contrast is monitored within the blood vessels and all the other surrounding tissue. A perfusion map is built to measure several key physiological parameters for the brain such as the mean transit time and blood volume. Since the head is supposed to be an object fixed in space, any movement during data acquisition can negatively affect the CT images acquired therefrom.
Therefore, it would be desirable to design an apparatus and method that enables CT imaging for moving objects with a circular source locus, and CT imaging for a still object with a complicated source locus.